Method for selective bandlimited data acquisition in subsurface formations

ABSTRACT

The method for exploring desired characteristics of a subsurface sector, having at least one resonant frequency, is based on selectively transmitting suitable narrowband energy waves into the subsurface sector, thereby producing narrowband signals reflected off the subsurface sector. The transmitted narrowband energy waves can be selectively and optimally adjusted in real time so as to provide optimum illumination of the desired characteristics from the explored sector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Application No. 11/451,571filed June 13, 2006, which is incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED R/D

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of seismic dataacquisition. More specifically, the invention relates to methods forselective bandlimited data acquisition in real time, resulting inimproved imaging of economically valuable or useful earth targets ofinterest.

2. Background Technology

Surface acoustic sources generate seismic waves from the surface andoperate at relatively low frequencies resulting in low-resolutionsurveys. A few downhole seismic sources have been developed whichtransmit acoustic waves into the formation through a borehole medium.These downhole sources can operate at a higher frequency than surfacesources but often do not generate enough energy to result in accuratesurveys.

Conventional downhole sources include:

the cylindrical bender source using piezoelectric rings bonded to a tubedeveloped by Southeast Research Institute and described in Balogh etal.'s, “New Piezoelectric Transducer for Hole-to-Hole SeismicApplications,” 58th Annual International Meeting of the Society ofExploration Geophysics (1988), Session DEV2.5; the electro-acoustictransducer cylindrical bender source developed by Kompanek and describedin U.S. Pat. No. 4,651,044; the arc discharge pulse source developed bySouthwestern Research Institute as described in U.S. Pat. No. 5,228,011;the swept frequency borehole source developed by Western Atlas anddescribed in Owen et al.'s, “Arc Discharge Pulse Source for BoreholeSeismic Applications,” 58th Annual International Meeting of the Societyof Exploration Geophysics (1988), Session DEV2.4; the potential energy“drop mass” source developed by Institut Francais du Petrole (IFP) asdescribed in U.S. Pat. No. 4,505,362; the hammer launched sourcedeveloped by OYO Corporation and described in Kennedy et al.'s “ASwept-Frequency Borehole Source for Inverse VSP and Cross-BoreholeSurveying,” 7.sup.th Geophysical Conference of the Australian Society ofExploration Geophysics (1989), Volume 20, pages 133-136; and the orbitalvibrator developed by Conoco and described in Ziolkowksi et al.'s“Determination of Tube-Wave to Body-Wave Ratio for Conoco BoreholeOrbital Source,” 69th Annual International Meeting of the Society ofExploration Geophysics (1999), pages 156-159.

A few sources have been developed that are clamped against the boreholewall. These sources are generally more difficult to design, and not manyhave been developed. One source utilizes a hydraulic vibrator clampedagainst the borehole wall to oscillate a reaction mass axially orradially and is more fully described in Turpening et al.'s “Imaging withReverse Vertical Seismic Profiles Using a Downhole, Hydraulic, AxialVibrator,” 62nd Meeting of EAGE (2000), Session P0161.

Some of the most recent and promising techniques for improved imaginguse complicated mathematics, such as Fourier transforms, to deconstructthe seismic data into discrete frequencies. As is well known, a Fouriertransform utilizes windows, which suffer from the time-resolutionfrequency-localization tradeoff.

When the harmonic frequency of the desired target of interest isdisplayed, the image becomes much clearer than the broadband seismicimage. This resonance effect is described in The Leading Edge,Interpretational Applications of Spectral Decomposition in ReservoirCharacterization, Greg Partyka, 1999.

Recently, the more advanced technique of using wavelet transforms, whichmitigate the significant windowing problems associated with Fouriertransforms, has been successfully applied to geophysical problems.

Patent application 20050010366 of John Castagna describes the techniqueof Instantaneous Spectral Analysis, which decomposes the seismic signalfrom the time domain to the frequency domain by superimposing members ofa preselected “wavelet dictionary” onto the trace, cross-correlating,and subtracting the energy of the wavelets until some predefined minimumthreshold is reached. The result is a spectrum for each time location onthe trace. More on this subject can be found in “The Leading EdgeInstantaneous Spectral Analysis”, John Castagna, 2003. Partyka, G. A.,Gridley, J. A., and Lopez, J. A., 1999, Interpretational aspects ofspectral decomposition in reservoir characterization: The Leading Edge,18, 353-360. Castagna, J. P., Sun, Shenjie, and Siegfried, R. W., 2003,Instantaneous spectral analysis: Detection of low-frequency shadowsassociated with hydrocarbons, 120-127. Marfurt, K. J. and Kirlin, R. L.,2001, Narrow-band spectral analysis and thin-bed tuning: Geophysics, 66,1274-1283. The results obtained from data deconstruction are essentiallybased on mathematical estimates.

Another recent Industry development is time-lapse production imagingcommonly termed 4D seismic monitoring. It is a method of viewing thereservoir with repeat surveys to determine how it is drainingdynamically. Essentially, the seismic surveys are repeated with as muchprecision as possible in order to generate data sets that differ onlywith respect to changes associated with reservoir production. By findingthe residual between the time-lapse images, one is able to infersubsurface fluid flow patterns and place constraints on fluid conduitsand baffles associated with the drainage, thereby enabling one to modifyreservoir models and future drilling plans. Because these changes can besubtle, any improvement of the signal-to-noise ratio would have abeneficial effect for 4D monitoring.

Accurate repositioning of the seismic source is considered one of themost critical elements in achieving 4D monitoring precision.Furthermore, a priori knowledge of the source signature would bebeneficial. More on the subject can be found in the book “4D ReservoirMonitoring and Characterization” by Dr. Rodney Calvert.

The frequency range that is providing a given image is governed by theseismic wavelet, which initially represents the source signature andthen changes as it experiences a number of earth-filtering effects,including absorption, geometrical spreading, and scattering. Betterknowledge of the seismic source improves processes that remove the earthfiltering effects.

Additional Related prior art can be found in the following:

6,985,815 January 2006 Castagna et al, 6,661,737 December 2003Wisniewski et al, 5,093,811 March 1992 Mellor et al, 6,619,394 September2003 Soliman et al, 200,200,700,17 June 2002 Soliman et al, 5,077,697December 1991 Chang, 5,418,335 May 1995 Winbow, 5,371,330 December 1994Winbow, 200,500,757,90 April 2005 Taner, M. Turhan et al, 6,814,141November 2004 Huh et al, 200,201,486,06 October 2002 Zheng, Shunfeng etal, 200,201,793,64 December 2002 Bussear, Terry R et al,

Variable Frequency Seismic Sources

U.S. Pat. Nos. 4,014,403, 4,049,077, 4,410,062, 4,483,411 and 4,578,784issued to Joseph F. Mifsud describe tunable frequency land and marineseismic vibrators.

U.S. Pat. No. 4,014,403 relates to a vibrator in which the frequency ofvibration changes as the stiffness of a spring is automaticallyadjusted. As a result, the impedance of the spring resonates with theimpedance of the reaction mass to maximize the reaction impedance,thereby increasing the operating efficiency of the vibrator.

U.S. Pat. No. 4,049,077 shows the use of a coupling plate as feedbackfor controlling the vibrator operation. At low frequencies, the feedbackis proportional to the coupling plate position, and at higherfrequencies, the feedback is proportional to the coupling platevelocity.

U.S. Pat. No. 4,410,062 shows a compliant member whose compliance issuch that it is substantially rigid at the natural frequency of thevibrator, and the natural frequency of the driven load of the vibratoris within the seismic spectrum but is higher than the natural frequencyof the vibrator.

U.S. Pat. No. 4,483,411 shows a seismic source, which produces a varyingFM signal at the low end of the acoustic spectrum. The seismic sourceuses stiff oscillating radiators to create a signal in the water. Theseradiators are attached to devices acting as springs with a variablespring rate. Variation of the spring rate as a function of the frequencypermits the device to be tuned for maximum power output.

U.S. Pat. No. 4,578,784 shows a seismic source, which produces a varyingFM signal generally within the 10-100 Hz region of the spectrum.

U.S. Pat. No. 5,146,432 describes a method of characterizingtransducers, and the use of a characterized transducer in themeasurement of the impedance of cement located behind a section of acasing in a borehole.

U.S. Pat. No. 6,928,030 describes a seismic defense system having aclosely monitored seismic source used to relay vital information fromthe source to the receiver.

U.S. Pat. No. 6,661,737 describes a tool including a programmableacoustic source that is controlled by a computer. The tool is used forlogging.

Resonance

U.S. Pat. No. 5,093,811 refers to a fracture study technique in whichresonance is established in the borehole to investigate fracturedimension by comparing the standing wave response at the wellhead to themodeled response.

U.S. Pat. Nos. 5,137,109 and 6,394,221 refer to seismic sources thatsweep through a range of frequencies, the first utilizing hydraulicpressure to vary the resonance frequency of the device itself, and thesecond utilizing a series of variable frequency impacts to sweep theseismic range. Both are concerned with the seismic source itself and notwith adjusting the source output to reach a resonant frequency of thetarget of interest.

U.S. Pat. No. 5,239,514 refers to a tool having frequencies in the500-1500 Hz range, equivalent to a seismic band of 10-30 Hz, whichincludes much of the typical seismic band. Longer source intervals andstacking are used to increase energy and the signal-to-noise ratio. Thistool does not adjust the source output to reach a resonant frequency ofthe target of interest in order to increase the signal-to-noise ratio,nor does it use multiple narrowbands.

U.S. Pat. Nos. 4,671,379 and 4,834,210 describe a tool that creates astanding resonant pressure wave whose frequency depends on the spacingbetween two end means in a borehole. Frictional, structural, andradiated acoustic energy loses are compensated for by continuedapplication of pressure oscillations. This tool relies on establishingresonance at the source and not at the target of interest.

U.S. Pat. No. 5,081,613 describes a method that generates pressureoscillations that produce resonant frequencies in the wellbore. Afterremoving the effects of known reflectors, the resonant frequencies areused to determine the depth and impedance of downhole obstructions.While this method does take advantage of resonance, it is confined tothe wellbore and not to the target of interest.

As attested by the above references, the geophysical industry hasstruggled, and continues to struggle, to develop improved dataacquisition techniques for improved imaging, as well as for better andeasier characterization of targets of interest that are economicallysuitable for production, and for guidance in selecting optimum welllocations with reduced investments.

SUMMARY OF THE INVENTION

It is a primary object of this invention to address some of the priorart limitations mentioned above by extending the capabilities of knownseismic sources and methods for data acquisition. The method of thisinvention for exploring desired characteristics of a subsurface sectoris based on selectively transmitting suitable narrowband energy wavesinto the subsurface sector, thereby producing narrowband signalsreflected off the subsurface sector. An operator in the field canselectively adjust the transmitted narrowbands in real time so as toextract optimum illumination of the desired characteristics from theexplored sector. Also, the narrowband signals reflected off thesubsurface sector are preferably collected and recorded on location inreal time, as well as displayed on a monitor in real time for assistingthe operator to continue making the necessary frequency adjustments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a illustrates an embodiment of this invention in which abandlimited source and a receiver array are both positioned on the earthsurface.

FIG. 1 b illustrates another embodiment of this invention in which abandlimited source is positioned in a borehole and a receiver array ison the earth surface.

FIGS. 2 a and 2 b illustrate an impingement of bandlimited waves on topand base of a target of interest for a normal-incidence reflection.

FIGS. 3-8 show plots of the response amplitude of a target of interestvs. frequency.

FIG. 3 shows a plot of the fundamental odd frequency as a member of aset of odd harmonics.

FIG. 4 shows a plot of the fundamental even frequency as a member of aset of even harmonics.

FIG. 5 compares the frequency response of two different targets withdifferent thicknesses.

FIG. 6 shows the advantage in terms of response amplitude of anarrowband centered on a resonant frequency over a narrowband centeredon a distortion frequency.

FIG. 7 shows the advantage of using a narrowband centered on a resonantfrequency, which captures the peak broadband amplitude, over using abroadband, which includes distortion frequencies.

FIG. 8 shows a potential target of interest, which could be a thin sandtarget between two shale formations.

FIG. 9 is a schematic block diagram of an apparatus using the method ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

Defined Terms

-   “target of interest” is a subsurface geological unit of economic    interest,-   “target” means target of interest,-   “formation” is a general subsurface geological unit that is not    necessarily considered a target of interest,-   “sector of interest” is a part of the target of interest,-   “source” is a unit that supplies energy such as acoustic energy,-   “source”, and “transmitter” are used interchangeably,-   “receiver” is an acoustic-to-electric converter that receives    acoustic energy,-   “array” is a collection of sources, receivers, or any other grouping    of devices arranged for a specific purpose,-   “real time” means work in process,-   “resonance” means increased amplitude of reflection of an object    subjected to energy waves by the source at or near its own natural    frequency of constructive interference,-   “distortion” means decreased amplitude of reflection of an object    subjected to energy waves by the source at or near its own natural    frequency of destructive interference,-   “resonant frequency” means a frequency at which resonance occurs,-   “distortion frequency” means a frequency at which distortion occurs,-   “harmonic” means any resonant frequency,-   “fundamental frequency” is the lowest non-zero resonant frequency,-   “period of resonance” is the range of frequencies between two    resonant frequencies or distortion frequencies,-   “narrowband” is a range of frequencies significantly less than the    period of resonance of the target of interest at the fundamental    frequency,-   “broadband” is a range of frequencies greater than a narrowband,-   “bandlimited” means narrowband,-   “resolution” means the ability to separate two features, such as    closely spaced reflection interfaces,-   “trace” is a record of received seismic signals,-   “stack” is a composite record made by combining different records,-   “Interactive” means adjusting the acoustic source in real time    typically based on data received from the receivers,-   “Impedance” means the product of density and velocity, and-   “reflection coefficient” means the ratio of the amplitude of the    reflected wave to that of the incident wave. Note: a low impedance    layer over a high impedance layer will produce a positive    reflection, and a high impedance layer over a low impedance layer    will produce a negative reflection.

Description of the Method of the Invention

In FIGS. 1 a and 1 b source 101 and receiver array 103 are shownutilized on earth surface 104, or in an open borehole 113 of anyorientation, which is preferably a vertical or near-vertical borehole.

FIG. 1 a illustrates an embodiment in which bandlimited source 101 andreceiver array 103 are placed on surface 104.

FIG. 1 b illustrates an embodiment in which a bandlimited source 101 isplaced in wellbore 113 and receiver array 103 is placed on surface 104.

In use, source 101 transmits bandlimited vibratory waves 105 into ground106, which, after reflecting off the interfaces of target of interest107, are sensed or measured by appropriately positioned receivers 102 inarray 103.

When source 101 is activated, it generates downgoing vibratory waves 105within narrowbands, which propagate through underground formations 106to target of interest 107.

Reflections of these waves from interface 108, between upper formation106 a and target of interest 107, and interface 109 between target ofinterest 107 and lower formation 106 b, return as bandlimited upgoingwaves 110 to receivers 102 at the surface.

In the preferred embodiment, a sector of interest 111 can be studiedusing an array of receivers 103, which process the received bandlimitedupgoing waves 110. From the receivers they are utilized as inputs toadjust source 101 through feedback loop 112.

FIGS. 2 a and 2 b illustrate the impingement of bandlimited waves 105 inFIG. 1 on top 201 and base 202 of the target of interest for anormal-incidence reflection.

FIG. 2 a shows the impingement of a sinusoid having a period equal totwice the target thickness upon the two interfaces. Frequency f is equalto the inverse of the period or f=1/t, where t is the period of thewave. Assuming a low impedance target, with a deflection to the right(FIG. 2 a) being positive and equal and opposite coefficients ofreflection, the wave 203 reflected from top 201 and the wave 204reflected from base 202 is shown side-by-side. In this case trough 205from top reflected wave 203 aligns with trough 206 from bottom reflectedwave 204, yielding constructive interference.

FIG. 2 b shows the impingement of a sinusoid having a period equal tothe target thickness upon the two interfaces. Once again, the wave 203reflected from top 201 is shown side-by-side with the wave 204 reflectedfrom base 202. In this case, trough 207 from top reflected wave 203aligns with peak 208 from bottom reflected wave 204, thereby creatingdestructive interference.

FIG. 3 shows a plot of amplitude vs. twice the product of frequency fand target thickness T with odd harmonics, which occur for the case ofreflection coefficients with opposite sign. In this example, thereflection coefficients are also equal in magnitude. Destructiveinterference occurs at odd integer values of twice the product offrequency and target thickness. The plot shows fundamental odd frequency301 as a member of the set of odd harmonics 302 that repeat at everypoint fT=n+½, where n is a real positive integer or zero. The odddistortion frequencies 303 repeat according to fT=n.

FIG. 4 shows a plot of amplitude vs. twice the product of frequency andtarget thickness with even harmonics, which occur for the case ofreflection coefficients with the same sign. In this example, thereflection coefficients are also equal in magnitude. Constructiveinterference occurs at even integer values of twice the product offrequency and target thickness. The plot shows fundamental evenfrequency 401 as a member of the set of even harmonics 402 that repeatat every point fT=n, where n is a real positive integer or zero. Theeven distortion frequencies 403 repeat according to fT=n+½.

In practice, most reflection coefficient pairs will not be equal inmagnitude, in which case they can be decomposed into even and oddcomponents. Also, the number of harmonics that are actually useful forimaging is generally small and depends strongly on the signal-to-noiseratio.

FIG. 5 shows a dual plot of amplitude vs. frequency and illustrates thethickness-dependant frequency response of two different targets. Theperiod of resonance P is equal to the inverse of the target thickness orP=1/T where T is the target thickness. Thus, thicker targets show asmaller resonance period. The response period for a 10 ms thick target501 is compared with the response period for a 50 ms thick target 502.

FIGS. 6 and 7 show plots of the amplitude of the response of target 107in FIG. 1 vs. frequency for an odd pair, where T=20 ms. Filteringeffects are neglected for emphasis.

FIG. 6 illustrates the substantial difference in the response amplitudefor a bandlimited signal 601 centered on a resonant frequency at 25 Hzas opposed to a bandlimited signal 602 centered on a distortionfrequency at 50 Hz. Accordingly, the signal-to-noise ratio will be muchgreater for the bandlimited signal centered on the resonant frequency.

FIG. 7 shows the increased average amplitude of the target response fora 20-30 Hz narrowband signal 701 centered on a resonant frequency f=25Hz over the target response for a 10-60 Hz broadband signal 702.Accordingly, the signal-to-noise ratio will be greater for thebandlimited signal centered on the resonant frequency than for abroadband signal.

FIG. 8 shows a possible target of interest, which could be a thin sandtarget encased in shale. The uppermost formation 801 and the lowermostformation 803 enclose a thin layer 802.

FIG. 9 shows a seismic apparatus 900 having a bandlimited seismic source901 optimally positioned next to a receiver 902 on earth surface 904.When source 901 is activated, it transmits downgoing vibratorynarrowband waves into the ground. After reflecting off sectors ofinterest 911, these waves return as bandlimited upgoing waves toreceiver 902 at the surface.

The signals generated by receiver 902 are passed to a signal conditioner908, which amplifies, filters and converts the analog signals to digitalsignals. The resulting digital signals are passed to a processor 909,which converts them into image signals.

The digital image signals from processor 909 are passed to imagingmeans, illustrated as a display 910, which can be a conventionalblack-and-white or color monitor. The digital signals from processor 909are also passed to a digital data collector 912.

In operation, an operator of apparatus 900 controls the output signalsfrom source 901 through a signal adjuster 905 that is designed tocontrol the source and its energy output.

The operator evaluates the images presented on display 910 and decideswhether the video signals have been optimized. If the answer is yes, theacquisition data is stored in data collector 912 and data collectioncontinues. If the answer is no, the operator uses signal adjuster 905 toadjust the output of source 901 until the image on display 910 moreclosely approximates the optimal harmonic resonance response expectedfrom sector of interest 911.

A memory unit (not shown) in processor 909 stores information indicativeof the received bandlimited return signals, which can be furtherprocessed depending on future needs. A communication device (not shown)in apparatus 900 can allow for direct communication with remotelylocated control units.

Using a feedback loop from processor 909 to adjuster 905, the desiredsource output adjustments could be executed automatically, therebyallowing the operator to intervene only as a troubleshooter.

Sources

Conventional sources in the above mentioned prior art include surfaceacoustic sources, downhole seismic sources, swept frequency boreholesources, tunable frequency land and marine seismic vibrators,feedback-controlled vibrators, orbital vibrators, programmable acousticsources that are controlled by a computer, sources that are clampedagainst the borehole wall, and others.

The preferred seismic energy source for practicing the method of thisinvention is a controlled-frequency adjustable acoustic source capableof transmitting frequencies within narrowbands. It can be positioned onthe surface or Inside a borehole. It can be conveyed into an openborehole by any known means such as production tubing, coiled tubing,cable, wireline, etc.

The source may produce bandlimited vibratory waves either simultaneouslyor sequentially, which can be held constant for some predeterminedduration, or can be varied incrementally. When the source is activatedit transmits vibratory waves into the ground within narrowbands, which,after reflecting off the targets of interest, are sensed and measured bythe appropriately positioned receivers.

Receivers

A conventional receiver has long been a velocity measuring geophone.However, accelerometers are becoming more widely utilized, andmulti-axis, or multi-component, accelerometers are emerging.Multi-component three axis sensing has produced superior images of thesubsurface as compared to single component sensing.

Receivers provide signals indicative of the sensed seismic energy to anacquisition device that can be co-located with the receiver unit andcoupled thereto for receiving the signal. A memory unit is disposed inthe acquisition device for storing information indicative of thereceived signal. A communication device may also be co-located with thereceiver/acquisition unit for allowing direct communication with aremotely located control unit.

In the crosswell or interwell seismic technique, the source is placed ina borehole and the receivers are placed in adjacent boreholes. Whenusing a reverse vertical seismic profiling technique, the source isplaced in a borehole and the receivers are placed along the surface asshown in FIG. 1 b. In the long spacing sonic technique, both the seismicsource and the receiver are placed in the same borehole. The crosswelltechnique is preferred. Both the source and the receivers can also beplaced on the surface as shown in FIG. 1 a.

Bandlimited Data Acquisition

When correlated to the harmonics of a specific target of interest, eachreceived bandlimited segment will have improved accuracy over broadbandcollection due to elimination of many waves that are not conducive toimaging, such as those created by uncontrolled seismic energy sources.

Through the production of energy within selected multiple narrowbands,the details of individual geologic targets of interest becomeaccentuated. This is because each target of interest responds optimallyto energy produced within specific narrowbands centered on harmonics.

Harmonic resonance occurs when the bandlimited reflections from twointerfaces are in phase as shown in FIG. 2 a, thereby producing anamplified reflection that is the sum of the reflection coefficients.

Harmonic distortion occurs when the bandlimited reflections from the twointerfaces are 180 degrees out of phase as shown in FIG. 2 b, in whichcase the amplitude will be the difference between the reflectioncoefficients. If the reflection coefficients are equal, harmonicdistortion results in complete destruction of the signal.

If the reflection coefficients are equal in magnitude and opposite insign, the response will show odd harmonics as shown in FIG. 3. If thereflection coefficients are equal in magnitude and equal in sign, theresponse will show even harmonics as shown in FIG. 4. In the generalcase, the response will be some combination of these two components, inwhich case the larger component will dominate.

Once the fundamental frequency of a given target of interest has beendetermined by adjusting the frequency of the source, other harmonicswill occur at a period that is the inverse of the thickness of thetarget of interest.

A target of interest with a given thickness will respond preferentiallyto energy produced at one set of harmonics, while a target of interestwith another thickness will show a peak response to energy produced atanother set of harmonics as shown in FIG. 5.

The central or peak frequency of the bandlimited waves applied by thesource should be appropriate for the depth of penetration necessary toimage the target of interest.

The range of narrowbands will occur within the range of seismicfrequencies, which is generally between 10 Hz and 250 Hz, although thisrange can vary depending on the source and other specific imagingconditions.

Bandlimited collection of data can focus on a single target of interestor multiple targets of interest. For example, the data collection effortmay focus on a petroleum reservoir, or on a petroleum reservoir togetherwith the surrounding or encasing formations, or stacked petroleumreservoirs, each of which may have a distinct optimal narrowband forimaging.

Real Time Bandlimited Data Acquisition

Based on information received by the receivers, real time interactivefrequency adjustments to the source can be made by an operator or by afeedback loop so as to induce harmonic resonance within the targets ofinterest.

By utilizing the harmonic response properties of the target of interestto make real time adjustments to the narrowband signals, the target ofinterest can be quickly and optimally imaged.

For example, if a target of interest is more optimally illuminated byone narrowband than by an adjacent narrowband, generally the narrowbandwith the superior response is closer to the harmonic resonance of thetarget of interest.

The speed with which accurate subsurface images can be obtained is oftencrucial to operations in the oilfield. Decisions involving theexpenditure of vast sums of money are often necessarily made on shortnotice due to practical considerations, such as equipment schedulingand/or downtime.

Real time bandlimited data acquisition of the present invention enablesan operator to interact directly during the data collection process.

The novel method reduces processing expenses significantly by permittinginteractive real time adjustments to acquisition parameters thatoptimize target of interest response. Utilization of the narrowbandproducing harmonic resonance of the target of interest can reduce theprocessing time and inaccuracies inherent in current spectraldecomposition methods, which can produce large volumes of data.

By focusing the acquisition on the naturally occurring harmonicresonance of the target of interest, significant non-pertinent data canbe eliminated from consideration.

Eliminating the non-pertinent data intrinsically improves both theaccuracy of the data and the speed with which a quality subsurface imagecan be produced. This also permits the tailoring of data acquisition andprocessing to the requirements of each unique application by reducingthe volume of non-pertinent data.

The entirety of the data collected using the method of this invention,including that which is not used for immediate application, can bestored and made available for future analysis involving otherapplications, which are presently known or which may be developed atsome future date.

Narrowbands are collected independently of each other. However, if thesignal-to-noise ratio is high at multiple harmonics, the narrowbands canbe combined in ways that optimize the imaging of the subsurface target.Simply adding the time series of narrowbands centered on distinctharmonics will produce a more resolved image.

Thus, when used in conjunction with traditional stacking methods, thesignal-to-noise ratio can be increased by narrowband imaging whilemaintaining resolution by combining multiple narrowbands.

Accordingly, a target of interest can be imaged at harmonics byinputting much less energy into the ground than would otherwise berequired by the use of an uncontrolled energy source.

Earth-filtering Effects

Earth filtering effects can modify and degrade the seismic signal. Bymaking on the spot frequency adjustments in real time while knowing theseismic source, earth filtering effects can be better estimated andremoved.

Traditionally, earth-filtering effects are removed by applyingmathematical processes designed to remove these effects to the receivedseismic signal. Knowledge of the original bandlimited source signatureprovides additional constraints on the overall estimation offrequency-dependant earth filtering effects.

Spectral Information

Time-lapse reservoir 4D monitoring simply repeats former surveyspecifications, both in terms of source and receiver location and, forthe method of this invention, source frequency ranges.

In one application, spectral information together with the Instantaneousknowledge of the source signature may be used to guide selection ofsubsequent acquisition parameters for time-lapse monitoring, savingprocessing time and cost.

For a specific target, the need to estimate the narrowband parameters ofthe source is eliminated after the initial data collection effort. Thus,a priori knowledge of the optimal source signature parameterscorresponding to the harmonics of the target of interest will improveaccuracy and save time.

The angle of incidence of the reflection received from a given point ona target of interest is determined by the vertical position of the toolin the wellbore, the depth and orientation of the formation, theposition of the receiver, and the physical parameters of the subsurface.

In one application, if the narrowband data is collected at variousdepths in adjacent wellbores, frequency-dependant AVO data can becollected. AVO stands for amplitude variation with offset.

AVO techniques known in the art provide estimates of acoustic and shearwave impedances for the media on either side of a reflecting interface,which are dependent on the parameters of the target of interest,including lithology, porosity, and pore fluid content. These estimatesare based on various approximations to the Zoeppritz formulation ofreflection coefficient variation as a function of incidence angle.

By collecting data within narrowbands, AVO attribute analysis isimproved. For example, utilization of frequency-dependant AVO attributeseliminates the need for bandwidth balancing.

Using “real” data as opposed to mathematically deconstructed data, thecurrent method provides improved imaging, thickness estimation, andfrequency-dependant AVO.

It is also anticipated that the method of this invention will improvethe quality of the estimates of attenuation for gas reservoirs.

The aforementioned techniques can be implemented on conventionalprocessing software, the bandlimited nature of the signal being the onlydifference from conventional processing inputs.

Noise & Signal-to-noise Ratio

In the art of geophysical imaging, which includes the acquisition andprocessing of data, the primary factor limiting the quality of seismicimages is that of noise. The presence of noise in the seismic datadiminishes the interpretability of the image.

More noise results in substandard image quality, which can obscure thetarget of interest. It is therefore highly desirable to mitigate theseproblems by increasing the signal-to-noise ratio.

There are different types of noise, and they can be dealt with indifferent ways. On the processing side, noise can be reduced byalgorithmic data processing. Filtering in the frequency-wave numberdomain can reduce ground roll. Frequency filtering can also reducerandom noise, although the filter can also affect the signal.

The most powerful technique used in geophysics for cancellation ofrandom noise is that of stacking. In this technique, reflections from acommon midpoint are added together to increase the signal. Because thenoise is random, it is out of phase and statistically tends to cancelwhen added together.

On the acquisition side, ground role or surface wave noise is suppressedby positioning the receivers so that the relative responses of theindividual receivers to the surface wave energy cancel each other out.This is an example of coherent noise reduction.

In accordance with this invention, the signal-to-noise ratio is improvedthrough utilization of the periodically repeating resonance response ofthe target of interest, which is determined by the target thickness andreflection coefficient ratio.

In the plot of amplitude vs. frequency, the signal-to-noise ratio issimply the ratio of the area under the curve of the signal to that ofthe area under the curve of the noise.

Because random noise tends to be white or flat across the spectrum,regions of the spectrum centered on resonant frequencies of the targetof interest will have a higher signal-to-noise ratio than those centeredon the distortion frequencies as shown in FIG. 6.

Also, regions of the spectrum centered narrowly on resonant frequencieswill have a higher signal-to-noise ratio than a broadband signal, asshown in FIG. 7.

Therefore, focusing the data acquisition on the regions in the vicinityof the resonant frequencies maximizes the signal-to-noise ratio, aspreviously described.

The following expressions give the area under the curve for a plot ofamplitude of reflectivity vs. frequency for a typical target ofinterest, which might be a sand encased in shale as in FIG. 8, and

with thickness T=20 ms and an odd reflection coefficient pair r1=−0.1,and r2=1, utilizing

a broadband signal f=10-60 Hz, and

a bandlimited signal f=20-30 Hz

$\begin{matrix}{{\int_{10}^{60}{\left\lbrack {2\; r_{o}{\sin\left( {\pi\;{fT}} \right)}} \right\rbrack\ {\mathbb{d}f}}} = {{\left( {2\; r_{o}} \right)\left\{ {{\left\lbrack {{- \frac{1}{\pi\; T}}{\cos\left( {\pi\;{fT}} \right)}} \right\rbrack\left( {f = 60} \right)} - {\left\lbrack {{- \frac{1}{\pi\; T}}{\cos\left( {\pi\;{fT}} \right)}} \right\rbrack\left( {f = 10} \right)}} \right\}} = 6.37}} & (a) \\{{\int_{20}^{30}{\left\lbrack {2\; r_{o}{\sin\left( {\pi\;{fT}} \right)}} \right\rbrack\ {\mathbb{d}f}}} = {{\left( {2\; r_{o}} \right)\left\{ {{\left\lbrack {{- \frac{1}{\pi\; T}}{\cos\left( {\pi\;{fT}} \right)}} \right\rbrack\left( {f = 30} \right)} - {\left\lbrack {{- \frac{1}{\pi\; T}}{\cos\left( {\pi\;{fT}} \right)}} \right\rbrack\left( {f = 20} \right)}} \right\}} = 1.97}} & (b)\end{matrix}$

If the signal-to-noise ratio for the 10-60 Hz case is 10, then the noiselevel will be 0.637.

Assuming white noise, the noise level for 20-30 Hz will be 0.137,yielding a signal-to-noise ratio of 1.971/0.137=14.38.

Thus, by the method of this invention, the use of a narrowband in thisexample has improved the signal-to-noise ratio by about 44%.

Advantages & Benefits in Time & Investment

Some of the advantages of the selective real time bandlimited methodinclude without limitation:

It provides an improved image without the necessity of detailed spectraldecomposition analysis.

It can also be used in a marine setting.

The seismic source is controllable and frequency ranges can be tuned tothe target of interest response.

Because the source takes advantage of the target of interest harmonicresponse, it requires less input energy to generate a satisfactorysignal-to-noise ratio for imaging.

In addition to location of the source, the source signal parameters forthe specific target of interest will be known and more easilyrepeatable, thereby permitting more accurate 4D reservoir monitoring.Repeatability of multiple bandlimited investigations will provide moreinformation on fluid migration patterns and vastly improve accuracy.

The costs and inaccuracies associated with algorithmic processing ofseismic data are reduced.

The collection method reduces or eliminates noise associated withuncontrolled sources by not collecting it. This includes signals atdistortion frequencies not useful for imaging.

All publications, patents, and patent applications mentioned in thisspecification are herein incorporated by reference to the same extent asif each individual publication, patent, or patent application were eachspecifically and individually indicated to be incorporated by reference.

The descriptions given herein, and best modes of operation of theinvention, are not intended to limit the scope of the invention. Manymodifications, alternative constructions, and equivalents may beemployed without departing from the scope and spirit of the invention.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A method of seismic data acquisition for imaging seismiccharacteristics of at least one subsurface target sector of interesthaving at least one resonant frequency, comprising: a) sequentiallytransmitting multiple seismic narrowband energy waves throughunderground strata into the target sector of interest; b) receivingcorresponding sector-reflected seismic narrowband energy waves; and c)using the sector-reflected narrowband energy waves for inducing thetransmitted narrowband energy waves to approach a naturally occurringresonant response of the target sector of interest, wherein thenaturally occurring resonant response is based on reflections offinterfaces of the target sector of interest with other sectors; and d)using at least one narrowband selected on the basis of matching thetarget resonant response for imaging the seismic characteristics.
 2. Themethod of claim 1, wherein step c) comprises: A. using information fromthe reflected narrowband energy waves to adjust an energy source in amanner to transmit narrowband energy waves which more closely match thetarget resonant response, B. activating the energy source to transmit atleast one additional narrowband into the target sector of interest, andC. receiving additional information about the naturally occurringresonant response from returning reflected narrowbands, generated fromsteps A and B.
 3. The method of claim 2, wherein the energy source isadjusted by a human operator.
 4. The method of claim 2, wherein theenergy source is adjusted automatically using a feedback loop.
 5. Themethod of claim 1, wherein the seismic narrowband energy waves comprisewaves between about 10 Hz and about 250 Hz.